This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Sulphate of soda should be kept in well-stopped bottles. Large, colorless, transparent, monoclinic prisms, rapidly efflorescing on exposure to air, and ultimately falling into a white powder, odorless, having a cooling, saline and somewhat bitter taste, and a neutral reaction. Soluble in 2.8 parts of water at 15° 0. (59° F.), in 0.25 part of water at 33° C. (91.4° F.), and in 0.4 part of boiling water; insoluble in alcohol. Commercial sodium sulphate contains water in varying proportions; its solution should, therefore, be regulated by its specific gravity. Being easily soluble in water, solutions of 10.5 and 1 per cent, are made, or it is dissolved for immediate use. It must be remembered that in winter time, when the temperature is low, a 10 per cent, solution would separate crystals. It is, therefore, better to prepare for stock a solution either of 5 or 1 per cent. Dissolve either one or five parts by weight of the hydrous commercial sodium sulphate in either 99 or 95 parts of distilled water. Filter. The specific gravity of the filtered solution must be brought to 1.003 of the one per cent, and 1.020 of the five per cent-solution. This is important. Reduce if necessary by the addition of more water. Proportion 100 to 1, or 100 to 5.